Methods for regenerating lithium-enriched salt baths

ABSTRACT

Methods for regenerating poisoned salt bath comprising providing a salt bath comprising at least one of KNO3 and NaNO3, providing an ion-exchangeable substrate comprising lithium cations, contacting at least a portion of the ion-exchangeable substrate with the salt bath, whereby lithium cations in the salt bath diffuse from the ion-exchangeable substrate and are dissolved in the salt bath, and selectively precipitating dissolved lithium cations from the salt bath using phosphate salt. The methods further include preventing or reducing the formation of surface defects in the ion-exchangeable substrate by preventing or reducing the formation of crystals on the surface of the ion-exchangeable substrate upon removal from the salt bath.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. § 120 of U.S. application Ser. No. 15/777,422filed on May 18, 2018, granted on Feb. 11, 2020 as U.S. Ser. No.10/556,826 which in turn, claims the benefit of priority under 35 U.S.C.§ 371 of International Application Serial No. PCT/US2016/062664 filed onNov. 18, 2016, which in turn, claims the benefit of 35 U.S.C. § 119 ofU.S. Provisional Application Ser. No. 62/372,497 filed on Aug. 9, 2016,U.S. Provisional Application Ser. No. 62/272,903 filed on Dec. 30, 2015,and U.S. Provisional Application Ser. No. 62/258,114 filed on Nov. 20,2015, the contents of each of which are relied upon and incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The present disclosure generally relates to methods for regeneratinglithium-enriched salt baths. More particularly, the disclosure relatesto methods for using phosphate salts to remove lithium cations from saltbaths used in ion exchange processes for strengthening glass andglass-ceramic substrates while preventing or reducing the formation ofsurface defects.

BACKGROUND

Tempered or strengthened glass is often used in consumer electronicdevices, such as smart phones and tablets, due to its physical andchemical durability and toughness. In general, the durability oftempered glass and glass-ceramic substrates is increased by theincreasing amount of compressive stress (CS) and depth of layer (DOL) ofthe glass or glass-ceramic substrate. To provide a larger CS and deepenthe DOL, ion exchange processes may be used to strengthen glass orglass-ceramic substrates. In ion exchange processes, a glass orglass-ceramic substrate containing at least one smaller alkali metalcation is immersed in a salt bath containing at least one larger alkalimetal cation. The smaller alkali metal cations diffuse from the glasssurface into the salt bath while larger alkali metal cations from thesalt bath replace the smaller cations in the surface of the glass. Thissubstitution of larger cations for smaller cations in the glassgenerates a layer of compressive stress layer in the surface of theglass, thus increasing the resistance of the glass to breakage.

As the ion exchange proceeds, the salt concentration of the smalleralkali metal cations (i.e., the cations that diffuse from the glass intothe salt) increases while the salt bath concentration of the largeralkali metal cations (i.e., the cations that migrate into the glass fromthe salt) decreases. This fluctuation in the ion concentration may causeunwanted constituents to form, which can age or “poison” the salt bath,or cause salt crystals to form and adhere to the surface of the glass. Apoisoned salt bath will not produce a large CS and deep DOL in the glasssubstrate, as desired. Likewise, crystals on the surface of the glasscan form defects—including depressions and protrusions. Dimpled,stippled glass, and glass that is not properly strengthened is notcommercially desired and may be unsuitable for use in some industries

SUMMARY

Embodiments herein address these needs by providing methods forregenerating salt baths by selectively precipitating dissolved lithiumcations from the salt bath to prevent and remedy salt bath poisoningwhile preventing or reducing surface defects.

In embodiments, methods for regenerating a salt bath include heating asalt bath with a phosphate salt and at least one of KNO₃ and NaNO₃. Thesalt bath is heated to a temperature of greater than or equal to 360° C.to less than or equal to 430° C. and an ion-exchangeable substrate isbrought into contact with the salt bath. As the ion exchange processbegins, the lithium cations diffuse from the ion-exchangeable substrateand are dissolved in the salt bath. The phosphate salt selectivelyprecipitates the lithium cations from the salt bath, keeping theconcentration of dissolved lithium cations in the salt bath less than orequal to 2.0 weight percent (wt %) lithium.

In some embodiments, methods for regenerating a salt bath includecontacting a salt bath with from greater than or equal to 0.10 wt % toless than or equal to 5.0 wt % of a phosphate salt and at least one ofKNO₃ and NaNO₃ with an ion-exchangeable substrate. As the ion exchangeprocess begins, the lithium cations diffuse from the ion-exchangeablesubstrate and are dissolved in the salt bath. The lithium cationsdiffuse from the ion-exchangeable substrate at a rate of greater than orequal to 1,000 μm²/hr and less than or equal to 8,000 μm²/hr. Thephosphate salt selectively precipitates the diffused lithium cationsfrom the salt bath, keeping the concentration of dissolved lithiumcations in the salt bath less than or equal to 2.0 wt % lithium.

In embodiments, methods for regenerating a salt bath include heating asalt bath comprising at least one of KNO₃ and NaNO₃ to a temperature ofgreater than or equal to 360° C. and less than or equal to 430° C. Atleast a portion of a first ion-exchangeable substrate comprising lithiumis contacted with the salt bath, and lithium cations diffuse from theion-exchangeable substrate and dissolve in the salt bath. Thecompressive stress of the first ion-exchangeable substrate is measuredafter the first ion-exchangeable substrate is contacted with the saltbath. At least a portion of subsequent ion-exchangeable substratescomprising lithium are contacted with the salt bath, and again lithiumcations diffuse from the subsequent ion-exchangeable substrates anddissolve in the salt bath. The subsequent compressive stresses of thesubsequent ion-exchangeable substrates are measured after they arecontacted with the salt bath, and phosphate salt is added to the saltbath when the compressive stress of a subsequent ion-exchangeablesubstrate is 10 MPa to 70 MPa less than the compressive stress of thefirst ion-exchangeable substrate. Dissolved lithium cations areselectively precipitated from the salt bath, such that the concentrationof dissolved lithium in the salt bath is greater than or equal to 0 wt %lithium and less than or equal to 2.0 wt % lithium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically depicts a salt bath during an ion exchange processbefore lithium poisoning according to embodiments disclosed anddescribed herein;

FIG. 1B schematically depicts a salt bath during an ion exchange processafter lithium poisoning;

FIG. 2A schematically depicts a salt bath before regeneration of alithium-poisoned salt bath;

FIG. 2B schematically depicts a salt bath after regeneration of alithium-poisoned salt bath according to embodiments disclosed anddescribed herein;

FIG. 3A schematically depicts a salt bath during regeneration accordingto embodiments disclosed and described herein;

FIG. 3B schematically depicts a salt bath during regeneration accordingto embodiments disclosed and described herein;

FIG. 4 schematically depicts an ion-exchangeable substrate during saltbath regeneration according to embodiments disclosed and describedherein;

FIG. 5A schematically depicts an ion-exchangeable substrate in which aphosphate crystal has formed on the surface;

FIG. 5B schematically depicts a reaction occurring between a phosphatecrystal and an ion-exchangeable substrate;

FIG. 6 schematically depicts an ion-exchangeable substrate comprising asurface defect;

FIG. 7 schematically depicts a power, a pellet, and a capsule ofphosphate salt according to embodiments disclosed and described herein;

FIG. 8 is a contour plot of the compressive stress (CS) ofion-exchangeable substrates at different dosing concentrations of LiNO₃and disodium phosphate (DSP), according to embodiments disclosed anddescribed herein;

FIG. 9 is a graph of the compressive stress of glass articles after eachrun in a salt bath containing 1 wt % trisodium phosphate and a bathcontaining 0.5% silicic acid according to embodiments disclosed anddescribed herein;

FIG. 10 is a graph of the compressive stress of glass articles aftereach run in a salt bath containing 1 wt % trisodium phosphate followingregeneration of the salt bath with the addition of 2 wt % trisodiumphosphate according to embodiments disclosed and described herein;

FIG. 11 is a graph of the compressive stress of glass articles aftereach run in a salt bath containing 1 wt % trisodium phosphate comparedto the compressive stress of glass articles after each run in a saltbath containing 2.5 wt % disodium phosphate and 0.5 wt % silicic acidaccording to embodiments disclosed and described herein;

FIG. 12 is a graph of the concentration of LiNO₃ present in two saltbaths in which one bath contains trisodium phosphate and the othercontains disodium phosphate according to embodiments disclosed anddescribed herein;

FIG. 13 is a graph of the surface profile of an ion-exchangeablesubstrate having a surface defect, as measured using Zygo 3D imaging andsurface metrology;

FIG. 14 is a second graph of the surface profile of a defect-freeion-exchangeable substrate, as measured using Zygo 3D imaging andsurface metrology, according to embodiments shown and described herein;

FIG. 15 is a graph of a high temperature x-ray diffraction analysis of amolten salt bath containing KNO₃, NaNO₃, and a small amount of lithiumsodium phosphate according to embodiments shown and described herein;

FIG. 16 is a graph of the effective speed of the phosphate salt powder,pellet, and capsule, according to embodiments shown and describedherein; and

FIG. 17 is a graph of the pH of a salt bath after adding various amountsof trisodium phosphate to the salt bath, according to embodiments shownand described herein.

DETAILED DESCRIPTION

Embodiments described herein are directed to methods for regeneratinglithium-poisoned salt baths used in ion exchange processes to strengthenlithium-containing glass and glass-ceramic substrates while preventingor reducing the formation of surface defects. The embodiments includeproviding a salt bath comprising at least one of KNO₃ and NaNO₃ and anion-exchangeable substrate comprising lithium cations, contacting atleast a portion of the ion-exchangeable substrate with the salt bath tocause lithium cations in the salt bath to diffuse from theion-exchangeable substrate and into the salt bath, and selectivelyprecipitating the dissolved lithium cations from the salt bath with aphosphate salt.

As used herein, the terms “ion exchange bath,” “salt bath,” and “moltensalt bath,” are, unless otherwise specified, equivalent terms, and referto the solution or medium used to effect the ion exchange process with aglass or glass-ceramic substrate, in which cations within the surface ofa glass or glass-ceramic substrate are replaced or exchanged withcations that are present in the salt bath. It is understood that in someembodiments the salt bath comprises at least one of KNO₃ and NaNO₃, maybe liquefied by heat or otherwise heated beyond a substantially solidphase.

As used herein, the terms “substrate” and “article” are, unlessotherwise specified, equivalent terms, referring to a glass orglass-ceramic material of any shape or form including, but not limitedto, sheets, vials, and three dimensional glass articles.

As used herein, the terms “cation” and “ion” are considered equivalentterms, unless otherwise specified. The terms “cation” and “ion” can alsorefer to one or more cations. While potassium and sodium cations andsalts are used in embodiments, it is understood that all embodiments ofthe disclosure are not limited to these species. The scope of thepresent disclosure also includes other metal salts and ions,particularly cations and salts of the alkali metals, as well as those ofother monovalent metals.

As used herein, the terms “selectively” and “selective” are used torefer to the affinity for a product or reaction mechanism to bepromoted, such that the particular product or reaction mechanism occursover other potential products or reactions.

As used herein, the term “diffusivity” refers to the molar flux of aspecies due to molecular diffusion or concentration gradient of theparticular species.

As used herein, the terms “granular” or “granulated” refers to acomposition comprising distinguishable pieces or grains with discretecomponents. [INVENTORS: PLEASE CONFIRM OR MODIFY THIS DEFINITION.]

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a” component includes aspects having two or moresuch components, unless the context clearly indicates otherwise.

Specific embodiments will now be described with reference to thefigures. The following description of the embodiments is illustrative innature and is in no way intended to be limiting in its application oruse. Furthermore, it should be understood that like reference numbersindicate corresponding or related parts in the various figures.

FIGS. 1A and 1B schematically depict a salt bath 100 during an ionexchange process before and after lithium poisoning. The salt bath 100contains larger alkali metal cations 120, which will be exchanged withsmaller alkali metal cations, depicted as lithium cations 130, presentin the ion-exchangeable substrate 105. As the smaller alkali metalcations in the ion-exchangeable substrate 105 are exchanged with thelarger alkali cations present in the salt bath 100, a layer ofcompressive stress is generated in the surface of the ion-exchangeablesubstrate 105. This compressive stress layer may inhibit both crackformation and crack propagation.

In some embodiments, the salt bath 100 may comprise at least one of KNO₃and NaNO₃. In some embodiments, the salt bath 100 may comprise acombination of KNO₃ and NaNO₃. The combination of KNO₃ and NaNO₃ may beselected based on the desired application. KNO₃—when compared toNaNO₃—comprises a larger alkali metal cation (i.e., K⁺ compared to Na⁺)that will more readily exchange with larger alkali metal cations in theglass substrate, such as Na⁺. Likewise, NaNO₃—when compared toKNO₃—comprises a smaller alkali metal cation (i.e., Na⁺ compared to K⁺)that more readily exchange with smaller alkali metal cations in theglass substrate. Accordingly, in some embodiments, the concentrations ofKNO₃ and NaNO₃ in the salt bath may be balanced based on the compositionof the ion-exchangeable substrate 105 to provide an ion exchange processthat increases both the CS and DOL in the ion-exchangeable substrate105. For example, the salt bath 100 may comprise from greater than orequal to 40 mol % and less than or equal to 95 mol % KNO₃, and fromgreater than or equal to 5 mol % and less than or equal to mol 60%NaNO₃. In some embodiments, the salt bath 100 may comprise from greaterthan or equal to 45 mol % to less than or equal to 50 mol % KNO₃, andfrom greater than or equal to 50 mol % to less than or equal to 55 mol %NaNO₃. In some embodiments, the salt bath 100 may comprise from greaterthan or equal to 75 mol % KNO₃ to less than or equal to 95 mol % KNO₃and from greater than or equal to 5 mol % NaNO₃ to less than or equal to25 mol % NaNO₃. In some embodiments, the salt bath 100 may comprisegreater than or equal to 45 mol % KNO₃ and less than or equal to 67 mol% KNO₃, and greater than or equal to 33 mol % NaNO₃ and less than orequal to 55 mol % NaNO₃.

The ion exchange process may be promoted, in some embodiments, byheating the salt bath 100. If the temperature of the salt bath 100 isincreased too much, it may be difficult to adequately control the ionexchange process and the DOL could increase too quickly withoutobtaining a good CS. Accordingly, the salt bath 100 may, in someembodiments, be heated to a temperature of greater than or equal to 360°C. and less than or equal to 430° C. In some embodiments, the salt bath100 may be heated to a temperature of greater than or equal to 360° C.to less than or equal to 390° C., or of greater than or equal to 375° C.to less than or equal to 385° C., or of greater than or equal to 378° C.to less than or equal to 383° C., or of greater than or equal to 410° C.to less than or equal to 430° C. Alternatively, the salt bath 100 may beheated to a temperature of greater than or equal to 400° C. to less thanor equal to 430° C., or greater than or equal to 425° C. to less than orequal to 435° C. The salt bath 100 may be heated to a temperature of380° C. or to a temperature of 420° C.

In some embodiments, before, during, or following the ion exchangeprocess, the salt bath 100 may have a pH of from less than or equal to10 and greater than or equal to 6, as measured by dissolving 5 wt % ofthe molten salt in aqueous solution and measuring the pH at roomtemperature (a temperature from 20° C. to 25° C., such as 21° C.). Insome embodiments, the salt bath 100 may be basic or alkaline, meaningthat the salt bath may have a pH of greater than 7 at 25° C. In someembodiments, the salt bath 100 may have a pH of from less than or equalto 10 to greater than or equal to 7.5 or from less than or equal to 10and greater than or equal to 8, or from less than or equal to 10 andgreater than or equal to 9, as measured by dissolving 5 wt % of themolten salt in aqueous solution and measuring the pH at roomtemperature.

Although FIGS. 1A and 1B show the ion-exchangeable substrate 105completely immersed in the salt bath 100, it should be understood thatin embodiments, only a portion of the ion-exchangeable substrate 105 iscontacted with the salt bath 100. The ion-exchangeable substrate 105 maybe brought into contact with the molten salt through immersion in a saltbath 100, or through spraying, dipping, or other similar means ofcontacting the ion-exchangeable substrate 105 with the salt bath 100. Insome embodiments, the ion-exchangeable substrate 105 may be fullysubmerged in the salt bath 100 or only a portion of the ion-exchangeablesubstrate 105 may be submerged in the salt bath 100. Theion-exchangeable substrate 105 may be brought into contact with the saltbath 100 multiple times, including but not limited to, dipping theion-exchangeable substrate 105 into the salt bath 100.

At least a portion of the ion-exchangeable substrate 105 may becontacted with the salt bath 100 for a period of time of from 1 minuteto 60 hours. At least a portion of the ion-exchangeable substrate 105may be contacted with the salt bath 100 for a period of time of from 1minute to 48 hours, or from 1 minute to 24 hours, or from 10 minutes to24 hours, or from 10 minutes to 2 hours. In some embodiments, theion-exchangeable substrate 105 may be contacted with the salt bath 100for from 5 hours to 40 hours, or from 5 hours to 20 hours, or for 8hours to 24 hours. In some embodiments, at least a portion of theion-exchangeable substrate 105 may be contacted with the salt bath 100for a period of time of from 10 minutes to 1 hour, or 10 minutes to 30minutes, or from 1 hour to 3 hours.

In embodiments, the ion-exchangeable substrate 105 is a glass orglass-ceramic substrate or article. The glass or glass-ceramic substratemay, in some embodiments, comprise alkali aluminosilicate or alkalialuminoborosilicate glass. For example, in some embodiments, theion-exchangeable substrate 105 may be formed from a glass compositionwhich includes greater than or equal to 50 mol % SiO₂ and less than orequal to 80 mol % SiO₂, greater than or equal to 0 mol % B₂O₃ and lessthan or equal to 5 mol % B₂O₃, greater than or equal to 5 mol % Al₂O₃and less than or equal to 30 mol % Al₂O₃, greater than or equal to 2 mol% Li₂O and less than or equal to 25 mol % Li₂O, greater than or equal to0 mol % Na₂O and less than or equal to 15 mol % Na₂O, greater than orequal to 0 mol % MgO and less than or equal to 5 mol % MgO, greater thanor equal to 0 mol % ZnO and less than or equal to 5 mol % ZnO, greaterthan or equal to 0 mol % SnO₂ and less than or equal to 5 mol % SnO₂,and greater than or equal to 0 mol % P₂O₅ and less than or equal to 10mol % P₂O₅. Alternatively, the ion-exchangeable substrate may comprisegreater than or equal to 60 mol % SiO₂ and less than or equal to 75 mol% SiO₂, greater than or equal to 0 mol % B₂O₃ and less than or equal to3 mol % B₂O₃, greater than or equal to 10 mol % Al₂O₃ and less than orequal to 25 mol % Al₂O₃, greater than or equal to 2 mol % Li₂O and lessthan or equal to 15 mol % Li₂O, greater than or equal to 0 mol % Na₂Oand less than or equal to 12 mol % Na₂O, greater than or equal to 0 mol% MgO and less than or equal to 5 mol % MgO, greater than or equal to 0mol % ZnO and less than or equal to 5 mol % ZnO, greater than or equalto 0 mol % SnO₂ and less than or equal to 1 mol % SnO₂, and greater thanor equal to 0 mol % P₂O₅ and less than or equal to 5 mol % P₂O₅. In someembodiments, the ion-exchangeable substrate 105 may not comprise B₂O₃,P₂O₅, MgO, ZnO, SnO₂, or combinations thereof. It should be understoodthat the aforementioned glass composition is one embodiment of a glasscomposition that may be used in the ion exchange process and that otherlithium containing glass compositions for use with the methods describedherein are contemplated and possible.

As mentioned, in embodiments, the ion-exchangeable substrate 105comprises lithium cations 130. In some embodiments, the ion-exchangeablesubstrate 105 may comprise greater than or equal to 2.0 mol % Li₂O andless than or equal to 25 mol % Li₂O. In other embodiments, theion-exchangeable substrate 105 may comprise from greater than or equalto 2.0 mol % Li₂O to less than or equal to 15 mol % Li₂O, or fromgreater than or equal to 2.0 mol % Li₂O to less than or equal to 10 mol% Li₂O, or from greater than or equal to 2.5 mol % Li₂O to less than orequal to 10 mol % Li₂O. In embodiments, the ion-exchangeable substrate105 may comprise from greater than or equal to 5.0 mol % Li₂O to lessthan or equal to 15 mol % Li₂O, or from greater than or equal to 5.0 mol% Li₂O to less than or equal to 10 mol % Li₂O, or from greater than orequal to 5.0 mol % Li₂O to less than or equal to 8 mol % Li₂O.

The amount of lithium present in the ion-exchangeable substrate 105 mayallow for the ion exchange process to occur at a faster rate whencompared to the ion exchange processes of other ion-exchangeablesubstrates that do not contain lithium. In some embodiments, lithium maymore readily diffuse from the glass or glass-ceramic substrate and intothe salt bath 100 than other alkali metal cations. In some embodiments,the ion-exchangeable substrate 105 may diffuse lithium cations 130 fromthe substrate at a rate of greater than or equal to 1,000 squaremicrometers per hour (μm²/hr). The ion-exchangeable substrate 105 maydiffuse lithium cations 130 from the substrate at a rate of greater thanor equal to 1,500 μm²/hr, greater than or equal to 1,800 μm²/hr, greaterthan or equal to 2,000 μm²/hr, greater than or equal to 2,200 μm²/hr, orgreater than or equal to 2,500 μm²/hr. In each of the above embodiments,the ion-exchangeable substrate 105 may diffuse lithium cations 130 at arate of less than or equal to 8,000 μm²/hr, such as less than or equalto 6,000 μm²/hr or less than or equal to 4,000 μm²/hr. Although thediffusion rate has been described using lithium, it should be understoodthat the diffusion rate of any alkali metal cation may be adjusted.

A fast diffusion rate of lithium cations 130 from the ion-exchangeablesubstrate 105 into the salt bath 100 may allow for a longer, and thusdeeper, exchange of ions between the ion-exchangeable substrate 105 andthe salt bath 100. The diffusion rate of lithium cations 130 in the ionexchange process may govern the rate at which the ion-exchangeablesubstrate 105 is strengthened (by increasing CS and DOL) and generallydecrease the time of the ion exchange process and/or the contact withthe salt bath 100 to achieve a specific CS and/or DOL. Therefore, insome embodiments, a high lithium cation 130 diffusion rate may bedesired to decrease the time required for the ion exchange process.However, in some embodiments, a low lithium cation 130 diffusion ratemay be desired to reduce or prevent the formation of surface defects onthe ion-exchangeable substrate 105, as the increased presence of ions inthe salt bath 100 due to the fast diffusion rate may tend to propagatethe formation of phosphate crystals, which may produce more numeroussurface defects, more visible surface defects, or both.

For example, in an embodiment, methods for reducing or preventing theformation of phosphate crystals, surface defects, or both, may includeutilizing an ion-exchangeable substrate 105 with a low ion diffusionrate. The formation of phosphate crystals often occurs at the end of theion exchange process after the concentration of lithium cations 130present in the salt bath 100 has dissipated. Ion-exchangeable substrates105 with low diffusion rates, such as a rate of less than or equal to8,000 μm²/hr may allow for a slower, more shallow ion exchange, whichmay reduce or prevent phosphate crystal 247 formation. Therefore, insome embodiments, the ion-exchangeable substrate 105 may have adiffusion rate of less than or equal to 8,000 μm²/hr to prevent theformation of phosphate crystals 247 and may be greater than or equal to1,000 μm²/hr to allow the ion-exchangeable substrate 105 to be properlystrengthened during the ion exchange process. The diffusion rate may befrom 1,000 μm²/hr to 6,000 μm²/hr, or from 2,000 μm²/hr to 6,000 μm²/hr,or from 3,000 μm²/hr to 5,000 μm²/hr. In some embodiments, a diffusionrate of 8,000 μm²/hr may achieve an ion exchange depth of layer of 400μm in 5 hours, which, in some embodiments, may be the shortest timeneeded to achieve proper strengthening in an ion exchange reaction. Insome embodiments, a diffusion rate of 1,000 μm²/hr may achieve an ionexchange depth of layer of 400 μm in 40 hours, which may, in someembodiments, be the longest time needed proper strengthening in an ionexchange reaction.

The diffusion rate may be measured based on the equation listed below:

$\begin{matrix}{{{Diffusion}\mspace{14mu} {Rate}\mspace{14mu} \left( {{\mu m}^{2}\text{/}{hr}} \right)} = {{Depth}\mspace{14mu} {of}\mspace{14mu} {Layer}\mspace{14mu} ({\mu m}) \times \frac{{depth}\mspace{14mu} {of}\mspace{14mu} {layer}\mspace{14mu} ({\mu m})}{{time}\mspace{14mu} {elapsed}\mspace{14mu} ({hr}) \times 3.92}}} & {{EQUATION}\mspace{14mu} 1}\end{matrix}$

As mentioned above, lithium may more readily diffuse from theion-exchangeable substrate 105 than other alkali metals. In someembodiments, sodium may diffuse from the ion-exchangeable substrate 105at a rate of from 100 μm²/hr to 800 μm²/hr, or from 100 μm²/hr to 600μm²/hr, or from 200 μm²/hr to 500 μm²/hr. Without being bound by anyparticular theory, the diffusion rate of the lithium cations 130, sodiumcations, or both, may be controlled to prevent poisoning and to reducesalt crystal formation in the salt bath 100. In some embodiments, toohigh of a concentration of lithium cations 130, sodium cations, or both,may cause the fresh molten salt 101 to become poisoned or to formunwanted phosphate crystals, which may cause functional and/or aestheticdefects.

Referring again to FIG. 1A, an ion-exchangeable substrate 105 immersedin a salt bath 100 is schematically depicted. In FIG. 1A, the salt bath100 is a molten salt bath comprising KNO₃ and NaNO₃, such as describedabove. The salt bath 100 has fresh molten salt 101 and includes largeralkali metal cations 120. The salt is considered to be “fresh” when thesalt bath 100 has not been poisoned by smaller alkali metal cations,such as lithium cations 130, which may diffuse from the ion-exchangeablesubstrate 105 and into the salt bath 100 to poison the fresh molten salt101. The larger alkali metal cations 120 may, in some embodiments,comprise potassium, sodium, or combinations thereof, which may havedisassociated from the KNO₃ and NaNO₃ present in the salt bath 100.

In some embodiments, phosphate salts such as trisodium phosphate may beadded to the salt bath 100 to precipitate out the excess lithium cations130 as the lithium cations 130 exchange out of the ion-exchangeablesubstrate 105. The phosphate salt may be present in the salt bath 100before the ion-exchangeable substrate 105 is contacted with the saltbath 100 or the phosphate salt may be added to the molten bath after atleast one ion-exchangeable substrate 105 has contacted the molten saltbath 100, or, in some embodiments, both. The phosphate salt mayprecipitate the lithium cations 130 to prevent the fresh molten salt 101from becoming poisoned by having too high of a lithium concentration.However, after precipitation of the lithium cations 130, the excessphosphate present in the salt bath 100 may form salt crystals, which mayadhere to the surface of the ion-exchangeable substrate 105. Thephosphate crystals may interact with the ion-exchangeable substrate 105,causing potassium cations, with may be preferably selected by thephosphate anions, to diffuse from the ion-exchangeable substrate 105,while the sodium cations diffuse into the ion-exchangeable substrate.The increased concentration of sodium cations in the ion-exchangeablesubstrate 105 may create a depression in the ion-exchangeable substrate105 caused by the volumetric difference between sodium cations andpotassium cations.

Without being bound by any particular theory, in some embodiments,surface defects, such as depressions and protrusions, may be caused bycrystals bonding to the ion-exchangeable substrate 105 following an ionexchange treatment. These surface defects may be visually observablewith or without the use of a magnifying lens (such as a microscope) or agreen light inspection process. The crystals may bond to theion-exchangeable substrate 105 and remain on the ion-exchangeablesubstrate 105 following removal from the salt bath 100. As the residualsalt from the salt bath 100 cools on the ion-exchangeable substrate 105,larger alkali metal cations 120 present in the crystals may exchangewith smaller alkali metals such as the lithium cations 130 present inthe ion-exchangeable substrate 105. As the alkali metals exchange,localized stress may be formed in the ion-exchangeable substrate 105,creating a dimensional difference in the surface of the ion-exchangeablesubstrate 105. As the crystals are removed from the ion-exchangeablesubstrate 105 (such as by washing the ion-exchangeable substrate 105with water), small depressions may remain in the surface of theion-exchangeable substrate 105 caused by the exchange of cations fromthe crystal into the substrate. If the surface of the ion-exchangeablesubstrate 105 is thin, such as a thickness of less than or equal to 100μm, when the crystals are removed from the ion-exchangeable substrate105, the voids left by the larger alkali metal cations 120 may be forcedthrough the surface of the ion-exchangeable substrate 105 to formprotrusions on the an opposite surface of the ion-exchangeable substrate105.

FIG. 1B schematically depicts a poisoned molten salt 102. A salt bath100 is considered to have been “poisoned,” when lithium cations 130 havediffused from the ion-exchangeable substrate 105 into the salt bath 100such that the poisoned molten salt 102 contains an increasedconcentration of lithium cations 130. Having lithium cations 130 presentin the salt bath 100 in a concentration greater than 2 wt % lowers theCS and DOL of the ion-exchangeable substrate 105 when compared to anion-exchangeable substrate 105 in a salt bath 100 having a concentrationof lithium cations of less than or equal to 2 wt %, such as fresh moltensalt 101, as the increase of lithium cations 130 in the molten saltretards the ion exchange process. More particularly, a salt bath maybecome poised with lithium cations over time such that the ion exchangecharacteristics of the bath degrade over time and the strengthcharacteristics (including CS and DOL) of the ion-exchangeablesubstrates treated in the salt bath vary, creating inconsistent productattributes over a manufacturing run.

Referring now to an embodiment shown in FIG. 2A, a salt bath 200 withpoisoned molten salt 202 contains more than 2 wt % lithium cations 230in the poisoned molten salt 202. The salt bath 200 contains metalcations 220 similar to the salt bath 100 and larger alkali metal cations120, as described above with reference to FIGS. 1A and 1B. In FIG. 2A, aphosphate salt 240 is added to the salt bath 200 to regenerate thepoisoned molten salt 202. As mentioned, the phosphate salt 240 may beadded before or after the ion-exchangeable substrate 205 is contactedwith the salt bath 200. The phosphate salt 240 comprises a cation and ananion that dissolves in the salt bath and disassociate to form PO₄ ⁻³anions and cations (including, but not limited to, sodium or potassiumions). The dissolved PO₄ ⁻³ anions present in the salt bath 200 reactwith and selectively precipitate the dissolved lithium cations 230,favoring a reaction with the lithium cations 230 over other potentialreactions, such as a reaction with sodium cations or potassium cationsin the salt bath 200. The selective precipitation reaction producesinsoluble Li₃PO₄ and Li₂NaPO₄ and LiNa₂PO₄, and generates additionalcations (including, but not limited, to sodium and potassium cations) inthe salt bath 200, which further aid in the ion exchange process betweenthe ion-exchangeable substrate 205 and the salt bath 200.

To effectuate the selective precipitation of the lithium cations 230, insome embodiments, the phosphate salt 240 is added to the salt bath 200so that the salt bath comprises greater than or equal to 0.10 wt % andless than or equal to 5.0 wt % of phosphate salt 240. In embodiments,the salt bath 100 may comprise from greater than or equal to 0.50 wt %to less than or equal to 5.0 wt %, from greater than or equal to 0.50 wt% to less than or equal to 1.0 wt %, or from greater than or equal to0.10 wt % to less than or equal to 5.0 wt % of phosphate salt 240. Inother embodiments, the salt bath 200 may comprise from greater than orequal to 0.10 wt % to less than or equal to 1.0 wt %, or from than orequal to 1.0 wt % to less than or equal to 5.0 wt % of phosphate salt240.

In some embodiments, the phosphate salt 240 may comprise Na₃PO₄, K₃PO₄,Na₂HPO₄, K₂HPO₄, Na₅P₃O₁₀, K₅P₃O₁₀, Na₂H₂P₂O₇, Na₄P₂O₇, K₄P₂O₇ Na₃P₃O₉,K₃P₃O₉, or combinations thereof. In some embodiments, the phosphate salt240 may comprise Na₃PO₄, or may comprise K₃PO₄, or may comprise acombination of Na₃PO₄ and K₃PO₄. In some embodiments, the phosphate salt240 may comprise anhydrous trisodium phosphate (Na₃PO₄), which maycontain 10% or less water and may have a chemical purity of at least 97%or greater. Trisodium phosphate may be commercially available astrisodium orthophosphate, anhydrous from Shifang Zhixin Chemical Co.Ltd., based in China, or Prayon Inc., based in the United States(Augusta, Ga.). In some embodiments, the concentration of phosphate inthe salt bath 200 may be greater than or equal to 50 parts per million(ppm) phosphate and less than or equal to 1,000 ppm phosphate. Theconcentration of phosphate in the salt bath 200 may be 500 ppm to 1,000ppm phosphate or 250 ppm to 750 ppm phosphate.

FIG. 2B shows an embodiment of a salt bath 200 with regenerated moltensalt 211. Particularly, the salt bath 200 has been regenerated by theaddition of the phosphate salt 240 consequently precipitating lithiumcations 230 out of the salt bath 200 in the form of insoluble lithiumphosphate (Li₃PO₄) 250. Accordingly, the concentration of lithiumcations 230 dissolved in the salt bath 200 is not greater than 2 wt % inthe regenerated molten salt 211. That is, the phosphate salt 240 hasselectively precipitated the lithium cations 230 to form insolublelithium phosphate 250, reducing the lithium cations 230 dissolved in thesalt bath 200 to less than 2 wt %. The dissociation of alkali metalcations (such as sodium or potassium) from the phosphate salt 240 infavor of the formation of lithium phosphate salt 250 also provides thesalt bath 200 with more dissolved metal cations 220 that can later beexchanged into the ion-exchangeable substrate 205. Regenerated moltensalt 211 is distinguished from the fresh molten salt 101 of FIG. 1A, inthat fresh molten salt 101 does not have lithium cations 130 present inthe salt bath 100, whereas regenerated molten salt 211 may have equal toor greater than 0 wt % but less than or equal 2.0 wt % lithium cations230 present in the salt bath 200. The lithium cations 230 present in thesalt bath 200 may react with the nitrate NO₃ ⁻ anions present in thesalt bath 200 to form lithium nitrate. In some embodiments, theconcentration of lithium nitrate in the salt bath 200 may be greaterthan or equal to 0 wt % and less than or equal to 2 wt %. In someembodiments, the lithium nitrate concentration may be less than 1.5 wt %or less than 1 wt %, such as from greater than or equal to 0 wt % andless than or equal to 1.5 wt % or less than or equal to 1 wt % in thesalt bath 200. In some embodiments, the lithium nitrate concentration inthe salt bath 100 may be greater than or equal to 0 wt % to less than orequal to 0.5 wt %, greater than or equal to 0 wt % to less than or equalto 0.1 wt %, or greater than or equal to 0 wt % to less than or equal to0.05 wt %. Alternatively, the lithium nitrate concentration in the saltbath 200 may be from greater than or equal to 0.5 wt % to less than orequal to 2 wt %, or from greater than or equal to 0.1 wt % to less thanor equal to 1.5 wt %.

In some embodiments, the phosphate salt 240 may be added to the saltbath 200 such that the concentration of phosphate salt 240 is less thanor equal to the concentration of the lithium cations 130 present in thesalt bath 200. Without being bound by any particular theory, excessphosphate salts 240 may, in some embodiments, generate crystals on thesurface of the ion-exchangeable substrate 105. These crystals maypropagate defects on the surface of the ion-exchangeable substrate 105,which may not be desirable. By maintaining a concentration of phosphatesalt 240 that is equal to or less than the concentration of the lithiumcations 130, the generation of surface defects may be reduced orprevented. This will be discussed further below with specific referenceto FIGS. 4 to 6.

In embodiments, the salt bath 200 may be used for at least 300 hoursbefore the regenerated molten salt 211 needs to be changed. In otherembodiments, the salt bath 200 may be used for at least 100 hours, 150hours, 175 hours, 200 hours, 250 hours, or 400 hours before theregenerated molten salt 211 needs to be changed.

As shown in the embodiments depicted by FIGS. 2A and 2B, the phosphatesalt 240 may be added to “spike” the salt bath 200 without theion-exchangeable substrate 205 present in the bath. For example, inembodiments, a poisoned salt bath may be spiked between ion exchangecycles. “Spiking” the salt bath 200 refers to adding phosphate salt to apoisoned salt bath. In embodiments, the phosphate salt 240 may be addedin amounts from greater than or equal to 0.10 wt % to less than or equalto 10.0 wt %. In some embodiments, the salt bath 200 may be spikedbefore the ion-exchangeable substrate 250 is contacted with the saltbath 200, or after the ion-exchangeable substrate is contacted with thesalt bath 200, or both. For instance, in some embodiments, a salt bath200 comprising at least one of KNO₃ and NaNO₃ is heated and contactedwith an ion-exchangeable substrate 205. The compressive stress of thefirst ion-exchangeable substrate 205 may be measured thought anytechniques known in the industry. As the ion exchange process proceeds,the lithium cations 130 may begin to diffuse from the ion-exchangeablesubstrate 205, poisoning the salt bath 200. As the salt bath 200 becomesincreasingly poisoned with the lithium cations 130, the compressivestress of subsequent ion-exchangeable substrates 205 produced bycontacting the substrate with the poisoned bath may begin to decrease ascompared to the compressive stress of ion-exchangeable substratesproduced by contacting the substrates with the non-poisoned bath. Oncesubsequently treated ion-exchangeable substrates 205 produced bycontacting the substrate with the poisoned salt bath 200 have acompressive stress below a certain value relative to theion-exchangeable substrates treated by contacting with a non-poisonedbath, phosphate salt 240 may be added to the bath. In some embodiments,the ion-exchangeable substrates 205 are removed from the salt bath 200,the salt bath 200 may be spiked with phosphate salts 240 to mitigate thepoisoning lithium cations 130 by selectively precipitating the lithiumin the bath into sludge, which may then be removed from the bath.

In some embodiments, the at least one ion-exchangeable substrate 205 maybe removed from the salt bath 200 when the compressive stress of thesubsequent ion-exchangeable substrate 205 produced by contacting thesubstrate to the poisoned salt bath is measured to be from 40 to 70 MPabelow the compressive stress of the first ion-exchangeable substrate 205initially produced by contacting the substrate with the non-poisonedsalt bath 200 before the lithium poisoning. In some embodiments, thephosphate salt may be added to the bath when the compressive stress ofsubsequent ion-exchangeable substrates measures to be 50 to 70 MPa belowthe compressive stress of the ion-exchangeable substrates 205 initiallyproduced by non-poisoned salt baths 200, or from 50 to 60 MPa, or from40 to 60 MPa below the compressive stress of the ion-exchangeablesubstrates 205 initially produced by non-poisoned salt baths 200.Similarly, if the salt bath 200 initially contained phosphate salt 240before the ion-exchangeable substrate 205 was contacted with the saltbath 200, a second phosphate salt may be added to the salt bath when thecompressive stress of the ion-exchangeable substrate 205 being treatedin the salt bath 200 containing diffused lithium cations 130 is from 40to 70 MPa less than a compressive stress of a substrate treated in asalt bath that does not contain diffused lithium cations. In someembodiments, the phosphate salt may be added when the compressive stressis 50 to 70 MPa below the compressive stress of the ion-exchangeablesubstrates 205 initially produced by non-poisoned salt baths 200, orfrom 50 to 60 MPa, or from 40 to 60 MPa below the compressive stress ofthe ion-exchangeable substrates 205 initially produced by non-poisonedsalt baths 200. In such embodiments, the second phosphate salt may havethe same composition or a different composition as the phosphate saltinitially contained in the salt bath.

In some embodiments, the phosphate salt 240 may be added to the saltbath 200 when the CS of the ion-exchangeable substrate 205 treated inthe poisoned salt bath 200 containing diffused lithium cations 130(hereinafter referred to as “second CS”) is from 10 MPa to 70 MPa belowthe CS of a similar ion-exchanged substrate in the non-poisoned saltbath 200 before the diffusion of lithium cations 130 (hereinafterreferred to as “second CS”). Lithium cations 130 may be selectivelyprecipitated from the salt bath 200 when the ion-exchangeable substrate205 has a compressive stress of from 10 MPa to 70 MPa less than thecompressive stress of a similar ion-exchangeable substrate 205 treatedin a salt bath 200 that does not contain diffused lithium cations. Toprecipitate the lithium cations 130, phosphate salt 240 may be added tothe salt bath 200 when the average second CS is approximately 50 to 60MPa, or 30 to 40 MPa, or 45 to 55 MPa below the first CS of theion-exchangeable substrate 205. In other embodiments, the phosphate salt240 may be added when the average second CS is approximately 20 to 30MPa, or 30 to 50 MPa, or 40 to 45 MPa, or 45 to 50 MPa below the firstCS.

In some embodiments, phosphate salt 240, such as TSP, may be added tothe salt bath 200 until the second CS is within 10 to 20 MPa of thefirst CS of the non-poisoned ion-exchangeable substrate 205. Withoutbeing bound by any particular theory, by not adding enough TSP to bringthe CS back to the original level in the non-poisoned ion-exchangeablesubstrate 205 may aid in reducing or preventing the formation ofphosphate crystals 247. In some embodiments, phosphate salts 240 may beadded until the second CS is within 15 to 25 MPa of the original CS ofthe ion-exchangeable substrate 205, or until the CS is within 5 to 10MPa, or within 15 to 20 MPa, or within 10 to 15 MPa, or within 15 to 25MPa of the first CS of the non-poisoned ion-exchangeable substrate 205.In some embodiments, phosphate salt 240 may be added to the salt bath200 to maintain a CS of from 10 to 50 MPa below the first CS of theion-exchangeable substrate 205. In some embodiments, phosphate salt 240may be added to the salt bath 100 to maintain a second CS of from 10 to40 MPa, or 20 to 50 MPa, or 20 to 40 MPa, or 15 to 45 MPa, or 5 to 40MPa below the first CS of the non-poisoned ion-exchangeable substrate205.

The amount of phosphate salts 240 added to the salt bath 100 may dependon a variety of factors, including the ion-exchangeable substrate 105and the size of the salt bath 100. In some embodiments, adding 0.5 wt %TSP may equate to a CS recovery of approximately 30 MPa. Therefore, insome embodiments, for a salt bath 100 that holds approximately 1400 kgof salt, 0.5 wt % of 1400 kg would be equivalent to 7 kg of TSP neededto recover the CS of the ion-exchangeable substrate 105 by an increasein CS of about 30 MPa. [INVENTORS: DO YOU HAVE ANY OTHER EXAMPLES?]

In some embodiments, before, during, and/or after the addition ofphosphate salt 240, the salt bath 100 may be inspected for visualquality. If the salt bath 200 is still cloudy, such as from the additionof phosphate salt 240, it may stain the ion-exchangeable substrate 205and may, in some embodiments, create a haze on the ion-exchangeablesubstrate 205. Hazy or stained ion-exchangeable substrates 205 may notbe suitable for use in particular industrial applications. In someembodiments, the salt bath 200 may be inspected for visual qualitybefore the ion-exchanged substrate 205 is contacted with the salt bath200 to prevent haze and/or staining.

In some embodiments, the phosphate salt 240 may be added to the saltbath 200 in a substantially solid phase before the ion-exchangeablesubstrate 205 is added to the salt bath 200. For instance, the phosphatesalt 240 may be added to the salt bath 200 as a powder, a pellet, acapsule, or in granular form. In some embodiments, the phosphate salt240 may be added to the salt bath 200 as a powder to promptly regeneratethe salt bath 200 due to the high surface-to-volume ratio of powders. Inother embodiments, the phosphate salt 240 may be added to the salt bath200 in a capsule form to promptly sink to the bottom of the salt bath200. The advantages and disadvantages of adding the phosphate salt 240in a powdered, pellet, and capsule form will be discussed further belowwith reference to FIGS. 7 and 16.

After the phosphate salt 240 is added to the salt bath 200, thephosphate salt 240 may settle in the salt bath 200. Some phosphate salt240 may substantially dissolve to a liquid phase in the salt bath 200,which may allow for stratification to occur between the lithium cations230 and the liquefied phosphate salt 240. In some embodiments, the saltbath 200 may comprise both liquid phosphate salt 240 which maydisassociate into PO₄ ⁻³ anions and cations, and solid phosphate salt240, which may be allowed to settle into the bottom of the salt bath 200before the ion-exchangeable substrate 205 is contacted with the saltbath 200, so as to prevent defects from forming in the ion-exchangeablesubstrate 205. In some embodiments, the disassociated phosphate salt 240may stratify and selectively precipitate the lithium cations 230. Inembodiments, as lithium phosphate 250 is selectively precipitated fromthe salt bath 200, the phosphate salt 240 may transition from asubstantially solid phase at the bottom of the salt bath 200 to asubstantially liquid phase, disassociating into PO₄ ⁻³ anions andcations to allow for further selective precipitation with the lithiumcations 230. Thus, it should be understood that the liquid phase of thephosphate salt 240 may react with the lithium cations 230 in a liquidphase to form insoluble (i.e., solid) lithium phosphate 250 which mayprecipitate from the salt bath 200.

Some embodiments further comprise forming insoluble lithium phosphate250 from the selective precipitation reaction, as discussed above, andremoving the insoluble lithium phosphate 250 from the salt bath 200. Insome embodiments, the lithium phosphates 250 may settle to the bottom ofthe salt bath 200, where the lithium phosphate 250 may be cleared orremoved with a filter, or a sieve or strainer, or removed through othermeans. Alternatively, the insoluble lithium phosphate 250 may remain inthe bottom of the salt bath 200.

Referring now to FIGS. 3A and 3B, a salt bath 300 is schematicallydepicted during regeneration. FIG. 3A depicts a salt bath 300 with freshmolten salt 301 that comprises phosphate salt 340 in addition to the atleast one of KNO₃ and NaNO₃. The ion-exchangeable substrate 305 containslithium cations 330 which will diffuse out of the ion-exchangeablesubstrate 305 and into the salt bath 300. As shown in FIG. 3B, due tothe phosphate salts 340 present in the fresh molten salt 301, as thelithium cations 330 diffuse from the ion-exchangeable substrate 305, thelithium cations 330 will be selectively precipitated by the phosphatesalt 340 to form insoluble lithium phosphate 350, which sinks to thebottom of the salt bath 300. The phosphate salt 340 selectivelyprecipitates the lithium cations 330 to a concentration of less than 2wt % in the salt bath 300, and the fresh molten salt 301 of FIG. 3Abecomes regenerated molten salt 311 as the lithium cation 330concentration is greater than or equal to 0 wt % and less than or equalto 2 wt % lithium cations 330.

As shown in FIGS. 3A and 3B, in some embodiments the salt bath 300 doesnot need to be spiked, as discussed above. In some embodiments, thephosphate salt 340 may be added to the salt bath 300 to form lithiumphosphate 350 as the lithium cation 330 concentration increases duringthe ion exchange process. This allows for the ion exchange process tooccur at a faster rate. Furthermore, it allows for the regeneratedmolten salt 311 to be changed less frequently.

Referring now to FIG. 4, a schematic view of an ion-exchangeablesubstrate 105 in a salt bath 100 is depicted. As phosphate salt 240 isadded to the salt bath 100, the lithium cations 130 present in the saltbath 100 are selectively precipitated to form lithium phosphates 250. Insome embodiments, the lithium phosphates 250 may be a mixed lithiumphosphate 242, such as lithium sodium phosphate, Li₂NaPO₄ or LiNa₂PO₄,or combinations thereof. Lithium cations 130 preferably bond with thephosphate salt 240 over other alkali metal cations present in the saltbath (such as the metal cations 220). However, as the lithium cation 130concentration diminishes in the bath, the phosphate salt 240 may beginto react with other alkali metal cations present in the salt bath, suchas medium alkali metal cations 140. The medium alkali metal cations 140may, in some embodiments, be larger than the lithium cations 130 butsmaller than the larger alkali metal cations 120. The medium alkalimetal cations 140 may be Na⁺, K⁺, or other alkali metals such as Rb⁺,Cs⁺, or Fr⁺.

Without being bound by any particular theory, as the lithium cations 130are precipitated out of the salt bath 100, the remaining phosphate salt240 present in the salt bath 100 may be attracted to the medium alkalimetal cations 140 to form phosphate alkali salts 246. The phosphatealkali salts 246 may comprise trisodium phosphate (“TSP,” Na₃PO₄) ortripotassium phosphate (K₃PO₄). The phosphate alkali salts 246 may bethe same or may be a different composition than the originally addedphosphate salts 240. In some embodiments, the phosphate alkali salts 246may not precipitate to the bottom of the salt bath 100 as lithiumphosphates 250 and phosphate alkali salts 246. Rather, in someembodiments, the phosphate alkali salts 246 may instead remain in thesalt bath 100.

Referring now to FIG. 5A, in some embodiments, the phosphate alkalisalts 246 form phosphate crystals 247, which may attach to the surfaceof the ion-exchangeable substrate 105, as depicted by arrow 249. Thephosphate crystals 247 may accumulate to form “sludge” in the salt bath100. The sludge may be a liquid slurry comprising phosphate crystals 247and other bath contaminants. The salt may be periodically analyzed todetermine the amount and content of the sludge in the salt bath 100. Insome embodiments, the sludge may be physically or chemically removedfrom the salt bath 100. For instance, the sludge may be physicallyskimmed out of the bath, or the sludge may be filtered from the bath.The formation of sludge may, in some embodiments, be proportional to theamount of phosphate salt 240 added to the salt bath 200. For instance,in some embodiments, 1 kg of TSP may produce about 0.8 kg of sludge.This weight discrepancy may be due to the exchange of sodium in the TSPfor lithium cations 130 which may precipitate to form Li₂NaPO₄, LiNa₂PO₄Li₃PO₄, of combinations thereof. In some embodiments, the rate at whichphosphate salt 240 is added to the salt bath 200 may be proportional tothe rate at which sludge may be removed from the salt bath 200.

Upon removal of the sludge, it may be necessary to add additional saltto the salt bath 100 to maintain the proper salt ratio in the bath. Insome embodiments, salt may be added to restore the ratio of the salts inthe salt bath 100 to the original salt ratio. For instance, if the saltbath originally contained 49 mol % NaNO₃ and 51 mol % KNO₃, and uponremoval of sludge some salt was lost from the bath such that the ratioof salt was now 43 mol % NaNO₃ and 57 mol % KNO₃, NaNO₃ may be added tothe salt bath to raise the content of NaNO₃ by 6 mol %. This may preventthe salt concentration from impacting the ion exchange process of theion-exchangeable substrate 205, which may cause unwanted changes in theproperties of the substrate.

Still referring to FIG. 5A, the medium alkali metal cations 140 (such assodium cations) present in the phosphate alkali salts 246 may begin toundergo an ion-exchange process with the ion-exchangeable substrate 105with the larger alkali metal cations 120 (such as potassium cations) andany remaining lithium cations 130 present in the ion-exchangeablesubstrate 105. The medium alkali metal cations 140 in the phosphatecrystals 247 may exchange into the surface of the substrate, while thelarger alkali metal cations 120 and any remaining lithium cations 130may exchange out of the substrate and into the phosphate crystals 247.While the lithium cations 130 may form insoluble lithium phosphate,Li₃PO₄, or insoluble lithium sodium phosphate, Li₂NaPO₄, or insolublelithium disodium phosphate, LiNa₂PO₄, and precipitate, the large alkalimetal cations 120 may form phosphate salts, (such as tripotassiumphosphate, K₃PO₄) which remain as a phosphate crystal 247.

In FIG. 5B, once the ion-exchangeable substrate 105 is removed from thesalt bath 100, the crystals are removed from the ion-exchangeablesubstrate 105, depicted by arrow 280. The crystals may be removed 280 bywashing the ion-exchangeable substrate 105 with water, such as deionizedwater, or by any known suitable method. The phosphate crystals 247 maynow contain the larger alkali metal cations 120 and lithium cations 130bonded with the phosphate salts 240 to produce, for instance, lithiumsodium phosphate Li₂NaPO₄ or lithium disodium phosphate LiNa₂PO₄ orpotassium sodium phosphate K₂NaPO₄ in the phosphate crystals 247. As thephosphate crystals 247 are removed, the ion-exchangeable substrate 105may be left with a surface depression where the phosphate crystals 247previously resided on the ion-exchangeable substrate 105.

As discussed previously, as the alkali metals exchange between theion-exchangeable substrate 105 and the phosphate crystals 247, thedifference in volumetric size between the larger alkali metal cations120, the medium alkali metal cations 140, and the lithium cations 130may generate localized stress in the ion-exchangeable substrate 105. Thelocalized stress may create a dimensional difference in the surface ofthe ion-exchangeable substrate 105 and may form a surface defect byleaving depression from the volumetric void previously occupied by thelarger sized cation. Ideally, ion-exchange process generates a uniformcompressive layer of stress in the ion-exchangeable substrate 105without regions of localized, highly concentrated areas of stress.Highly concentrated localized regions of stress may generate defects inthe ion-exchangeable substrate 105, including depressions andprotrusions in the surface of the ion-exchangeable substrate 105.

As shown in FIG. 6, the removal of the phosphate crystals 247 may revealsurface defects 440 on the surface of the ion-exchangeable substrate105, to form a defective ion-exchanged substrate 108, which may not bedesirable. Without being bound by any particular theory, in the surfaceof the ion-exchangeable substrate 105, the void from the larger alkalimetals (such as the larger alkali metal cations 120) may not be filledcompletely by the smaller alkali metals (such as the medium alkali metalcations 140), and as the phosphate crystals 247 are removed from theion-exchangeable substrate 105 small depressions in the surface of thedefective ion-exchanged substrate 108 may be formed. In someembodiments, a depression generated in one surface of a substrate mayproduce a protrusion on an opposite surface of the substrate. This mayparticularly occur in thin substrates, such as substrates having athickness of less than or equal to 100 μm, or less than or equal to 150μm, or less than or equal to 75 μm, or less than or equal to 50 μm.

In some embodiments, the surface defect 440 may be a depression or aprotrusion having a height or depth of greater than or equal to at least1.2 nm and a width or length of greater than or equal to at least 0.005mm. The surface defect 440 may have a height or depth of from 1 nm to200 nm, or from 1 nm to 100 nm, or from 1 nm to 10 nm. In someembodiments, the surface defect 440 may have a height or depth of from10 nm to 50 nm, or from 100 nm to 50 nm, or from 100 nm to 200 nm. Thesurface defect 440 may have a width or length of from 0.1 μm to 5 μm, orfrom 0.1 μm to 50 μm, or from 0.1 μm to 0.5 mm. In some embodiments, thesurface defect 440 may have a width or length of from 0.1 to 0.5 mm, orfrom 0.05 to 0.1 mm, or from 0.05 to 0.5 mm, or from 0.001 to 0.5 mm. Insome embodiments, the surface defects 440 may be visible to theunassisted eye. In other embodiments, the surface defects 440 may onlybe visible when placed under a green light or using a microscope oranother magnifying lens.

Various strategies may be employed to prevent or reduce the formation ofthe surface defects 440, the phosphate crystals 247, or both. In someembodiments, as previously mentioned, it may be desirable to limit theconcentration of phosphate salts 240 added to the salt bath 100 suchthat the concentration of lithium cations 130 is greater than or equalto the concentration of the phosphate salt 240 present in the salt bath100. As the phosphate salt 240 will preferably attract lithium cations130, which may precipitate out of the bath as solid lithium phosphates250, by limiting the concentration of phosphate salt 240 present in thebath to less than or the same as the concentration of the lithiumcations 130, the phosphate crystals 247 may not form, thereby preventingthe surface defects 440. While a minority of phosphate salt 240 maystill react with the larger alkali metal cations 120 and the mediumalkali metal cations 140 to form phosphate crystals 247, the reducedamount of phosphate crystals may be easily washed from theion-exchangeable substrate 105 without causing visible, or in someembodiments, any surface defects 440.

Another embodiment for reducing or preventing phosphate crystals 247,surface defects 440, or both, includes reducing the amount of time inwhich the ion-exchangeable substrate 105 is in contact with thephosphate salt 240. For instance, the formation of phosphate crystals247, surface defects 440, or both, may be reduced or removed by limitingthe amount of time in which the ion-exchangeable substrate 105 ispresent in the salt bath 100. This may prevent the formation of thephosphate crystals 247 in the salt bath 100, or may reduce theconcentration of the phosphate crystals 247 in the salt bath 100, bothof which may reduce the likelihood of the phosphate crystals 247attaching on the surface of the ion-exchangeable substrate 105.

Likewise, the formation of phosphate crystals 247, surface defects 440,or both, may be reduced or removed by quickly cooling theion-exchangeable substrate 105 upon removal from the salt bath 100. Insome embodiments, the ion-exchangeable substrate 105 may be cooled fromthe temperature of the salt bath 100 to a temperature of less than orequal to 100° C. in less than 5 minutes. In some embodiments, theion-exchangeable substrate 105 may be cooled to a temperature of lessthan or equal to 100° C. in less than 3 minutes, or less than 2 minutes,or less than 1 minute, or less than 1 minute and 30 seconds, or lessthan 30 seconds, or less than 10 seconds. Without intent to be bound byany particular theory, if the ion-exchangeable substrate 105 is cooleddown to a temperature of less than or equal to 100° C. in greater than 5minutes, the phosphate crystals 247 may be able to coalesce or otherwisegrow larger in size, which may propagate more surface defects 440, morevisible surface defects 440, or both. In some embodiments, theion-exchangeable substrate 105 may be cooled from a temperature ofgreater than or equal to 380° C. to a temperature of less than or equalto 100° C. in from 30 seconds to 3 minutes, or from 30 seconds to 1minute and 30 seconds. In some embodiments, the ion-exchangeablesubstrate 105 may be cooled to a temperature of less than or equal to100° C. in from 1 minute to 3 minutes, or from 1 minute and 30 secondsto 3 minutes, or from 45 seconds to 2 minutes or from 2 minutes to 3minutes.

Additionally, phosphate crystals 247 have a propensity to crystallize atlow temperatures due to their low solubility. In some embodiments, theformation of phosphate crystals 247, surface defects 440, or both may bereduced or prevented by utilizing a salt bath 100 with a temperature ofat least 380° C., such as at least 400° C., 420° C., or 450° C. Thetemperature of the salt bath 100 may be dependent on the composition ofthe ion-exchangeable substrate 105.

As previously mentioned, the method of introducing the phosphate salt240 to the salt bath 100 may affect the reactions between the lithiumcations 130 and the phosphate salt 240, thus impacting the formation ofthe phosphate crystals 247 or lack thereof. As shown in FIG. 7, thephosphate salt 240 may be added to the salt bath 100 as a powder 810, asa pellet 830, or as a capsule 850. As used here, a “powder” refers tofine dry particles produced by the grinding, crushing, or disintegrationof a solid substance. As used herein, a “pellet” refers to a compressedshaped mass of a powder. As used herein, a “capsule” refers to asubstance, such as a powder, that is substantially enclosed andenveloped by a sheath or membrane.

Conventionally, powdered substances may be preferred due to the highsurface-to-volume ratio that allows a powder 810 to react quickly andefficiently with the other reactants, such as the lithium cations 130.However, adding powder 810 to the salt bath 100 may result in a cloudybath in which the powder has stratified due to the small individualparticles, as powder may have a tendency to create dust duringdispensing that must be managed during application. In some embodiments,a salt bath 100 that is cloudy may not be suitable for an ion exchangeprocess, as it may impart various defects in the ion-exchangeablesubstrate 105. The salt bath 100 may remain cloudy for a long period oftime, such as 6 hours, 8 hours, 10 hours, one day or three days or more,prolonging the process and increasing the time and cost required toprocess the ion-exchangeable substrate 105. Conventionally, for thesereasons, a pellet 830 may be preferably used, as a pellet 830 may sinkto the bottom of the salt bath 100 without stratifying and producingcloudiness in the salt bath 100. However, a pellet 830 may not react asquickly with the lithium cations 130 due to the decreased surface areaof the pellet when compared to the powder 810.

Therefore, in some embodiments, a capsule 850 may be used to introducethe phosphate salt 240 to the salt bath 100. In some embodiments, thecapsule 850 may be comprised of powdered phosphate salts 240encapsulated with a salt mixture. In some embodiments, the salt mixturemay comprise NaNO₃, KNO₃, or both. Upon introduction to the salt bath100, the capsule 850 may sink to the bottom of the salt bath 100 wherethe encapsulated salt may dissolve, releasing the inner powderedphosphate salt 240 to quickly react with the lithium cations 130 withoutstratifying and causing a cloudy salt bath 100.

To prepare a capsule 850, in some embodiments, a phosphate salt may beblended with an encapsulating salt. The blended salts may be subjectedto heat at a temperature above the melting temperature of theencapsulating salt such that the encapsulating salt begins to fuse. Theheated salt mixture may be cooled to produce an encapsulated phosphatesalt, referred to as capsule 850.

Similarly, in some embodiments, granular TSP may be used. Granular TSPmay not produce as much dust during dispensing and may prevent the bathfrom otherwise clouding. Granulated TSP may have a coarse texture thatquickly settles in the bath, and in some embodiments, granular TSP maymore readily dissolve than the encapsulated powder based on the size andsurface area of the coarse particles. However, granular TSP may absorbmoisture faster than encapsulated TSP during storage.

In some embodiments, the phosphate salt may be a sodium phosphate, suchas trisodium phosphate or disodium phosphate. The encapsulating salt maybe a nitrate salt, such as potassium nitrate or sodium nitrate. Theencapsulating salt may be used in fine crystal, pellet, or granularform. In some embodiments, the encapsulating salt may have a lowermelting temperature than the phosphate salt, such that upon heating theencapsulating salt melts and fuses around the substantially non-meltedphosphate salt to create a capsule 850. In some embodiments, the meltedsalt may be cooled to room temperature (21° C.) or may be cooled at atemperature sufficient to form the capsule 850. For instance, the meltedsalt may be cooled to any temperature below the glass transitiontemperature (T_(g)) of the encapsulating salt. As shown below withreference to FIG. 17, in some embodiments, the capsule 850 may be ableto react with lithium cations 130 at substantially the same rate as thepowder 810.

The following examples illustrate one or more embodiments of the presentdisclosure as previously discussed above. The description of theembodiments is illustrative in nature and is in no way intended to belimiting it its application or use.

EXAMPLES Example 1—Lithium Poisoned Molten Salt of Sodium Nitrate

A molten salt of sodium nitrate (NaNO₃) containing 0.32 wt % of lithiumnitrate (LiNO₃) was characterized by inductively coupled plasma opticalemission spectrometry (ICP-OES). Into this salt, 1 wt % of sodiumphosphate (Na₃PO₄) was added to precipitate the lithium cations at 380°C. The treated molten salt was sampled and characterized by ICP-OES.Results showed that lithium cations were successfully removed frommolten salt (liquid phase). The concentration of LiNO₃ was below thedetection limit of ICP-OES (<0.005 wt %).

Example 2—Lithium Poisoned Molten Salt of Sodium Nitrate and PotassiumNitrate

The molten salt of 49 wt % of sodium nitrate (NaNO₃) and 51 wt % ofpotassium nitrate (KNO₃) contained 0.02 wt % of lithium nitrate (LiNO₃)characterized by ICP-OES. Into this salt, 1 wt % of trisodium phosphate(TSP, Na₃PO₄) was added to precipitate the lithium cations at 380° C.The treated molten salt was sampled and characterized by ICP-OES.Results showed that lithium cations were successfully removed frommolten salt (liquid phase). The concentration of LiNO₃ was below thedetection limit of ICP-OES (<0.005 wt %).

Example 3—Lithium Poisoned Molten Salt of Sodium Nitrate and PotassiumNitrate

Mixtures of 49 wt % of sodium nitrate (NaNO₃) and 51 wt % of potassiumnitrate (KNO₃) containing 0.5-1.0 wt % of lithium nitrate (LiNO₃) and0-2.5 wt % of disodium phosphate (DSP, Na₂HPO₄) were melted at 380° C.for 12 hours. Then, glass substrate (0.8 mm thickness,lithium-containing glass) were strengthened (ion-exchanged) in thesebaths for 3 hr. and 45 min. at 380° C. The strengthened glasses werecleaned and the compressive stress (CS) was measured by FSM (see Table1, below). The lithium concentrations (in terms of LiNO₃) of molten saltsamples were analyzed by ICP-OES (see Table 1, below).

Results showed that lithium cations can be effectively stratified by theaddition of phosphate (in this case, DSP). ICP data indicates lithiumcation concentrations can be significantly reduced within 12 hours at380° C. Using these baths for ion exchange, the CS was partially orcompletely recovered with the addition of DSP. Overdosing DSP helps afaster stratification process (see FIG. 8).

FIG. 8 is a contour plot of the compressive stress (CS) of ion exchangedglass substrates at different dosing concentrations of LiNO₃ anddisodium phosphate (DSP), as discussed in detail in Example 3. FIG. 8depicts five ion exchanged glass substrates with differing surfacecompressive stresses due to different ion exchange conditions (i.e., theconcentration of DSP and lithium nitrate salt in the bath). The five ionexchange glasses have compressive stresses of 445.0 megapascals (MPa),455.0 MPa, 465.0 MPa, 475.0 MPa, and 485.0 MPa. The contour plot showsthe dosing concentrations of LiNO₃ in wt % versus the concentration ofdisodium phosphate (DSP) in wt %. As can be seen from FIG. 8, thecompressive stress of the ion-exchanged substrates generally decreasesas LiNO₃ content increases.

TABLE 1 Stratification of Lithium Cations by DSP Soluble Average LiNO₃CS of the Added Added in bath after strengthened LiNO₃ DSP 12 hoursglass Conditions (wt %) (wt %) (wt %) (MPa) 1 0 0.5 0 495 2 0 0 0 490 30.5 0 0.43 450 4 0.5 0.5 0.3 465 5 0.5 0.5 0.22 465 6 1 0.5 0.72 440 70.5 2.5 0.05 487 8 1 2.5 0.13 470

For all conditions the concentration of KNO₃ and NaNO₃ were 51 wt %, and49 wt %, respectively.

Example 4—Trisodium Phosphate Bath Regeneration

To determine the effectiveness of trisodium phosphate (TSP, Na₃PO₄) inprecipitating lithium to regenerate a salt bath, 1 wt % TSP waspre-loaded in a 3 kg molten salt bath comprising 49 wt % NaNO₃ and 51 wt% KNO₃. The salt bath was maintained at a temperature of 380° C. and 25samples of Example Glass 1, a glass composition according to theembodiments described herein (composition described below in Table 2),were added to the bath. The glass pieces were 50 mm×50 mm×0.8 mm sheetsof glass, inserted into the salt bath for 3 hr. and 45 min. The ionexchange process was repeated for 15 runs. For purposes of comparison, asecond salt bath comprising 16 kg of 49 wt % NaNO₃ and 51 wt % KNO₃ with0.5 wt % silicic acid (Si(OH)₄ was melted to replicate a standardcommercial ion exchange bath. The second salt bath was maintained at atemperature of 380° C. and 120 samples of Example Glass 1 were added tothe bath and ion exchanged for 3 hr. and 45 min. The ion exchangeprocess was repeated for 6 runs. The compressive stress (CS) of theglass samples was determined after each run as a measure of the degreeof poisoning of the salt bath. That is, it was expected that thecompressive stress of the samples would decrease for each consecutiverun due to the increasing concentration of lithium in the bath as aresult of ion exchange.

FIG. 9 shows the compressive stress of the glass samples after each runof Example 4 for the bath containing 1 wt % TSP and the bath containing0.5 wt % silicic acid. As shown in FIG. 9, the bath containing 1 wt %TSP generally yielded higher compressive stresses than the bathcontaining 0.5 wt % silicic acid. The data also indicates that thedecrease in compressive stress in the bath containing 0.5 wt % silicicacid was more pronounced (i.e., the CS curve had a steeper slope) as thenumber of runs increased than the bath containing 1 wt % TSP. While notwishing to be bound by theory, it is believed that this trend is theresult of lithium poisoning in the bath containing 0.5 wt % silicic acidwhile the bath containing 1 wt % TSP at least partially mitigated theeffects of lithium poisoning.

In addition, to determine the effectiveness of regenerating the saltbath containing 1 wt % TSP, an additional 2 wt % TSP was added to thesalt bath containing TSP after run 15. This “spike” of TSP regeneratedthe salt bath by precipitating out the poisoning lithium cations. Asshown in FIG. 10, after run 15, when the additional 2 wt % TSP was addedto the salt bath, the compressive stress of the glass samples increasedfrom 460 MPa to 500 MPa. This demonstrates that, after regeneration, thesalt bath is able to produce glass pieces with similar compressivestresses as those in the original, fresh salt bath that has not beenpoisoned (i.e., as shown in FIG. 9).

Regeneration of the bath by the addition of TSP produced glass sampleswith compressive stresses over 440 MPa after 30 runs in the same saltbath. In prior ion exchange processes (i.e., without regeneration), thesalt bath would need to be changed after only 7-8 runs or batches. Byincreasing the time and the amount of batches of glass that may be ionexchanged before the molten salt bath must be cooled, cleaned, replacedand reheated, production efficiencies are improved and production costsare decreased.

TABLE 2 Composition of Example Glass 1 Example Glass 1 Mol % Oxide SiO₂63.46 Al₂O₃ 15.71 B₂O₃ 0 Li₂O 6.37 Na₂O 10.69 MgO 0.06 ZnO 1.15 P₂O₅2.45 SnO₂ 0.04

Example 5—Trisodium Phosphate Pre-Loading

To determine the amount of TSP that could be pre-loaded into the saltbath to prevent lithium poisoning, three salt bath tanks wereartificially poisoned with LiNO₃, as shown in Table 3 below. In thefirst salt bath, labeled “Tank 1,” 1.56 wt % of LiNO₃ was used toartificially poison a 3 kg bath comprising 49% NaNO₃ and 51% KNO₃. Thebath was maintained at a temperature of 380° C. 2.5 wt % TSP was addedto the poisoned bath. Once the constituents had settled in the bath, 25pieces of glass according to Example Glass 1 were added. The glasspieces were ion exchanged for 3 hr. and 45 min. The compressive stressin the glass was measured after ion exchange and determined to be 466.7MPa, an acceptable value for commercial production.

Similarly, in the second salt bath, labeled “Tank 2,” 6.25 wt % of LiNO₃was used to artificially poison a 3 kg bath comprising 49% NaNO₃ and 51%KNO₃. The bath was maintained at a temperature of 380° C. 10 wt % TSPwas added to the poisoned bath. Once the constituents had settled in thebath, 25 pieces of glass according to Example Glass 1 were added to thebath and ion exchanged for 3 hr. The compressive stress of the glass wasmeasured after ion exchange and determined to be 464.7 MPa with nocloudiness on the glass surface to indicate impurities.

The data in Table 3 demonstrates that large amounts of lithium poisoningmay be overcome by pre-loading up to 10 wt % TSP in the salt bath toprevent lithium poisoning. More specifically, the data demonstrates thatthe lithium poisoning can be overcome by maintaining the ratio of LiNO₃to TSP as the concentration of LiNO₃ in the bath increases withoutadversely affecting the compressive stress imparted to the glass. Thedata also confirmed that by pre-loading up to 10 wt % TSP into the saltbath, the life of the salt bath could be extended by at least 70 runswithout the need for regeneration.

TABLE 3 Compressive Stress using 2.5 wt % and 10 wt % TSP Number ofTank/Salt TSP LiNO₃/ LiNO₃ TSP CS Experiment Pieces Vol. (kg) (g) TSP(wt %)/ (ppm) (MPa) Artificially poisoned 25 3 75 0.62 0.28 130 466.7Tank 1, with 2.5% TSP, 1.56% LiNO₃ Artificially poisoned 25 3 300 0.620.23 <50 464.7 Tank 2, with 10% TSP, 6.25% LiNO₃

Example 6—Disodium Phosphate Bath Regeneration

To determine the effectiveness of disodium phosphate (DSP, Na₂HPO₄) inprecipitating lithium to regenerate a salt bath, 2.5 wt % DSP waspre-loaded in a 3 kg molten salt bath comprising 49 wt % NaNO₃ and 51 wt% KNO₃. The salt bath was maintained at a temperature of 380° C. and 25samples of Example Glass 1, a glass composition according to theembodiments described herein (composition described above), were addedto the bath. The glass pieces were 50 mm×50 mm×0.8 mm sheets of glass,inserted into the salt bath for 3 hours and 45 minutes.

FIG. 11 shows a comparison between Example 4 (TSP Regeneration), Example5 (DSP Regeneration), and silicic acid only (no phosphate addition). Asshown in FIG. 11, DSP is slightly less effective than TSP in salt bathregeneration, but is more effective than silicic acid alone.

TSP exhibited superior regeneration capability when compared to DSP overlonger periods of time, as shown FIG. 12. Particularly, FIG. 12 showsthe results of two salt baths of 49% NaNO₃ and 51% KNO₃ at 380° C. Bothsalt baths contained 0.495 wt % LiNO₃ pre-loaded to artificially poisonthe salt baths. To the poisoned salt bath of Tank 1, 0.37 wt % DSP wasadded. To the poisoned salt bath of Tank 2, 0.43 wt % TSP was added(equivalent to a 1.1 molar ratio of DSP and TSP). The wt % of LiNO₃ inthe bath was tested over a period of hours. While DSP and TSP showsimilar results after 5-10 hours, after 24 hours, TSP shows a decreasedlevel of LiNO₃ when compared to DSP. Further, after a period of 5 days(120 hours), the tank containing TSP has a lower concentration of LiNO₃when compared to the tank containing DSP.

Example 7—Surface Defects

FIG. 13 depicts a characterization of the surface defects present on aglass substrate following an ion exchange process in which the salt bathwas regenerated with trisodium phosphate salt (TSP). FIG. 13 is a graphof the Zygo measurements, a 3-dimensional (3D) imaging and surfacemetrology tool which depicts the surface profile of the glass sheet. Asthe ion exchange process occurred, the TSP began to precipitate out thelithium cations to form lithium phosphate salts. As the lithiumconcentration diminished, the excess TSP began to react with thepotassium present in the glass substrate, causing an influx of sodiumcations into the glass substrate, while phosphate crystals comprisingK₃PO₄ were formed. Upon removing the glass article from the bath andcooling the glass article, water was used to try to wash off thephosphate crystals. As the large phosphate crystals were removed fromthe glass substrate, surface defects were noticed on the glasssubstrate. As shown in FIG. 13, the surface profile of the glasssubstrate depicts both depressions and protrusions on the surface of theglass substrate. Additionally, some of the depressions were quite largein nature, up to 2 to 5 nm in depth, with most depressions showing alength of a few micrometers and a depth of a few nanometers.

Example 8—Surface Defects and Cooling Rate

FIG. 14 shows the Zygo reading for a glass substrate that was rapidlycooled upon removal from the bath. As previously discussed, rapidcooling may reduce or restrict the formation of phosphate crystals, andthus, surface defects, on the ion-exchangeable substrate. In Example 8,a glass article was ion exchanged in a 3 kg bath comprising 49% NaNO₃and 51% KNO₃. The bath was maintained at a temperature of 380° C. 10 wt% TSP was added to the bath to prevent and remedy lithium poisoning. Theglass article was removed from the bath and immediately cooled from atemperature of 380° C. to a temperature of less than 100° C. in oneminute. As shown in FIG. 14, the surface defects are much smaller inlength and width with very few protrusions.

Example 9—Surface Defects and Bath Temperature

FIG. 15 depicts an in-situ high temperature x-ray diffraction (XRD)pattern as a function of temperature and intensity for a molten saltbath containing KNO₃, NaNO₃, and a small amount of lithium sodiumphosphate. As shown in FIG. 15, at higher bath temperatures, such as380° C., the only crystals that were present in the molten salt bathwere lithium sodium phosphate, Li₂NaPO₄ salts, which remained in acrystalline form. At lower temperatures, the crystals from the othersalts (such as KNO₃, NaNO₃ and Li₃PO₄) were more prevalent. As themolten salt bath was heated to a temperature of 380° C. and then cooledto room temperature, above 300° C. only lithium sodium phosphateremained present in a crystalline state. Below 300° C., both when thebath was being cooled from 380° C. and when the bath was being heatedfrom room temperature, other crystalline phases existed from crystallinesalts present in the bath.

Example 10—Phosphate Salt Reaction Time

FIG. 16 depicts the differences between adding phosphate salt to amolten salt bath in a powdered, pellet, and capsule formulation, asdiscussed previously. In Example 10, trisodium phosphate was added tomolten salt baths in a powder form, a pellet form, and a capsule formfor a treatment time of 75 hours. To produce the capsules, 1 kg oftrisodium phosphate, 2 kg KNO₃, and 1.125 kg of NaNO₃ were mixedthoroughly in a stainless steel container. The mixture was placed in anoven and heated to a temperature of 390° C. The temperature of 390° C.was substantially maintained for 3 hours. The mixture was then removedand cooled to room temperature. Once cooled, the capsule was separatedfrom the stainless steel container.

As shown in FIG. 16, the encapsulated phosphate powder was able to reactin the molten salt bath with the lithium nitrate to precipitate outlithium phosphate salts at substantially the same rate as the powderedform. Contrastingly, the pellet form was not able to react as quickly,resulting in a higher concentration of lithium nitrate, present andunreacted in the bath. The capsule did not cloud the salt bath whilestill quickly and efficiently reacting with the lithium nitrate, makingthe capsule a superior choice to the powdered or pellet forms oftrisodium phosphate, or other encapsulated phosphate salts.

Example 11—Bath pH

FIG. 17 is a graph of the pH of the salt bath at varying amounts oftrisodium phosphate. In Example 11, a 3 kg bath comprising 49% NaNO₃ and51% KNO₃ was heated and maintained at a temperature of 380° C. Trisodiumphosphate (TSP) was added to the bath in accordance with the belowequation:

Na₃PO₄+1.75LiNO₃═Li_(1.75)Na_(1.25)PO₄+1.75NaNO₃  EQUATION 2

The pH was measured by dissolving 5 wt % of the salt in an aqueoussolution and measuring the pH at room temperature (approximately 20 to25° C.). In FIG. 17 the salt bath had a pH of from 6 to 10 as TSP isadded to the bath. Without the addition of TSP, the bath had a pH ofabout 7.8. Upon the addition of TSP, the pH of the bath initiallydropped to about 6.3. As more TSP was added to the salt bath, the pHrose to 7.6, then to 9.2, and then 9.8 as the concentration increased,as shown in FIG. 17.

It should be apparent to those skilled in the art that variousmodifications and variations can be made to the described embodimentswithout departing from the spirit and scope of the claimed subjectmatter. Thus, it is intended that the specification cover themodifications and variations of the various described embodimentsprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for regenerating a salt bath comprising:heating a salt bath comprising a phosphate salt and at least one of KNO₃and NaNO₃ to a temperature of greater than or equal to 380° C.;contacting at least a portion of an ion-exchangeable substratecomprising lithium cations with the salt bath, whereby lithium cationsdiffuse from the ion-exchangeable substrate and are dissolved in thesalt bath; and selectively precipitating, not during the contacting,dissolved lithium cations from the salt bath, wherein the concentrationof dissolved lithium in the salt bath is greater than or equal to 0 wt %lithium and less than or equal to 2.0 wt % lithium, and wherein theconcentration of lithium cations in the salt bath is greater than orequal to the concentration of phosphate salt in the bath.
 2. The methodfor regenerating a salt bath of claim 1, wherein the ion-exchangeablesubstrate comprises: greater than or equal to 50 mol % SiO₂ and lessthan or equal to 80 mol % SiO₂; greater than or equal to 0 mol % B₂O₃and less than or equal to 5 mol % B₂O₃; greater than or equal to 5 mol %Al₂O₃ and less than or equal to 30 mol % Al₂O₃; greater than or equal to0 mol % Na₂O and less than or equal to 15 mol % Na₂O; greater than orequal to 2.0 mol % Li₂O and less than or equal to 15 mol % Li₂O; greaterthan or equal to 0 mol % MgO and less than or equal to 5 mol % MgO;greater than or equal to 0 mol % ZnO and less than or equal to 5 mol %ZnO; greater than or equal to 0 mol % SnO₂ and less than or equal to 5mol % SnO₂; and greater than or equal to 0 mol % P₂O₅ and less than orequal to 10 mol % P₂O₅.
 3. The method for regenerating a salt bath ofclaim 2, wherein the dissolved lithium cations are selectivelyprecipitated by reacting with the phosphate salt thereby forming atleast one of insoluble Li₃PO₄, insoluble Li₂NaPO₄ or insoluble LiNa₂PO₄.4. The method for regenerating a salt bath of claim 3, wherein thephosphate salt is selected from the group consisting of Na₃PO₄, K₃PO₄,Na₂HPO₄, K₂HPO₄, Na₅P₃O₁₀, K₅P₃O₁₀, Na₂H₂P₂O₇, Na₄P₂O₇, K₄P₂O₇, Na₃P₃O₉,K₃P₃O₉, and combinations thereof.
 5. The method for regenerating a saltbath of claim 4, wherein the phosphate salt is added to the salt bath asan encapsulated powder.
 6. The method for regenerating a salt bath ofclaim 5, wherein the salt bath has a pH of less than or equal to 10, asmeasured by dissolving 5 wt % of the salt in aqueous solution andmeasuring the pH at room temperature.
 7. The method for regenerating asalt bath of claim 6, wherein the ion-exchangeable substrate has adiffusion rate of less than or equal to 8,000 μm²/hr.
 8. The method forregenerating a salt bath of claim 1, wherein the salt bath comprisesgreater than or equal to 40 mol % KNO₃ and less than or equal to 95 mol% KNO₃, and greater than or equal to 5 mol % NaNO₃ and less than orequal to 60 mol % NaNO₃.
 9. The method for regenerating a salt bath ofclaim 8, wherein heating further comprises heating the salt bath to atemperature less than or equal to 450° C.
 10. The method forregenerating a salt bath of claim 9, further comprising: removing theion-exchangeable substrate from the salt bath; and cooling theion-exchangeable substrate to a temperature of less than or equal to100° C. in less than or equal to 3 minutes.
 11. The method forregenerating a salt bath of claim 10, wherein the ion-exchangeablesubstrate comprises glass, glass-ceramic, or combinations thereof. 12.The method for regenerating a salt bath of claim 11, wherein the saltbath comprises greater than or equal to 75 mol % KNO₃ and less than orequal to 95 mol % KNO₃, and greater than or equal to 5 mol % NaNO₃ andless than or equal to 25 mol % NaNO₃.
 13. The method for regenerating asalt bath of claim 11, wherein the salt bath comprises greater than orequal to 45 mol % KNO₃ and less than or equal to 67 mol % KNO₃, andgreater than or equal to 33 mol % NaNO₃ and less than or equal to 55 mol% NaNO₃.
 14. A method for regenerating a salt bath comprising: heating asalt bath comprising at least one of KNO₃ and NaNO₃ to a temperature ofgreater than or equal to 380° C.; contacting at least a portion of afirst ion-exchangeable substrate comprising lithium with the salt bath,whereby lithium cations diffuse from the ion-exchangeable substrate andare dissolved in the salt bath; measuring a compressive stress of thefirst ion-exchangeable substrate after the first ion-exchangeablesubstrate is contacted with the salt bath; contacting at least a portionof subsequent ion-exchangeable substrates comprising lithium with thesalt bath, whereby lithium cations diffuse from the subsequention-exchangeable substrates and are dissolved in the salt bath;measuring subsequent compressive stresses of the subsequention-exchangeable substrates after the subsequent ion-exchangeablesubstrates are contacted with the salt bath; adding a phosphate salt tothe salt bath when a compressive stress of a subsequent ion-exchangeablesubstrate is from 10 MPa to 70 MPa less than the compressive stress ofthe first ion-exchangeable substrate; and selectively precipitating, notduring the contacting, dissolved lithium cations from the salt bath,wherein the concentration of dissolved lithium in the salt bath isgreater than or equal to 0 wt % lithium and less than or equal to 2.0 wt% lithium, and wherein the concentration of lithium cations in the saltbath is greater than or equal to the concentration of phosphate salt inthe bath.
 15. The method for regenerating a salt bath of claim 14,wherein the ion-exchangeable substrate comprises: greater than or equalto 50 mol % SiO₂ and less than or equal to 80 mol % SiO₂; greater thanor equal to 0 mol % B₂O₃ and less than or equal to 5 mol % B₂O₃; greaterthan or equal to 5 mol % Al₂O₃ and less than or equal to 30 mol % Al₂O₃;greater than or equal to 0 mol % Na₂O and less than or equal to 15 mol %Na₂O; greater than or equal to 2.0 mol % Li₂O and less than or equal to15 mol % Li₂O; greater than or equal to 0 mol % MgO and less than orequal to 5 mol % MgO; greater than or equal to 0 mol % ZnO and less thanor equal to 5 mol % ZnO; greater than or equal to 0 mol % SnO₂ and lessthan or equal to 5 mol % SnO₂; and greater than or equal to 0 mol % P₂O₅and less than or equal to 10 mol % P₂O₅.
 16. The method for regeneratinga salt bath of claim 15, wherein the dissolved lithium cations areselectively precipitated by reacting with the phosphate salt therebyforming at least one of insoluble Li₃PO₄, insoluble Li₂NaPO₄ orinsoluble LiNa₂PO₄.
 17. The method for regenerating a salt bath of claim16, wherein the phosphate salt is selected from the group consisting ofNa₃PO₄, K₃PO₄, Na₂HPO₄, K₂HPO₄, Na₅P₃O₁₀, K₅P₃O₁₀, Na₂H₂P₂O₂, Na₄P₂O₂,K₄P₂O₂, Na₃P₃O₉, K₃P₃O₉, and combinations thereof.
 18. The method forregenerating a salt bath of claim 17, wherein the phosphate salt isadded to the salt bath as an encapsulated powder.
 19. The method forregenerating a salt bath of claim 18, wherein the ion-exchangeablesubstrate has a diffusion rate of less than or equal to 8,000 μm²/hr.20. The method for regenerating a salt bath of claim 14, wherein thesalt bath comprises greater than or equal to 40 mol % KNO₃ and less thanor equal to 95 mol % KNO₃, and greater than or equal to 5 mol % NaNO₃and less than or equal to 60 mol % NaNO₃.
 21. The method forregenerating a salt bath of claim 20, wherein heating further comprisesheating the salt bath to a temperature less than or equal to 450° C. 22.The method for regenerating a salt bath of claim 21, further comprising:removing the ion-exchangeable substrate from the salt bath; and coolingthe ion-exchangeable substrate to a temperature of less than or equal to100° C. in less than or equal to 3 minutes.
 23. The method forregenerating a salt bath of claim 22, wherein the ion-exchangeablesubstrate comprises glass, glass-ceramic, or combinations thereof. 24.The method for regenerating a salt bath of claim 23, wherein the saltbath comprises greater than or equal to 75 mol % KNO₃ and less than orequal to 95 mol % KNO₃, and greater than or equal to 5 mol % NaNO₃ andless than or equal to 25 mol % NaNO₃.
 25. The method for regenerating asalt bath of claim 23, wherein the salt bath comprises greater than orequal to 45 mol % KNO₃ and less than or equal to 67 mol % KNO₃, andgreater than or equal to 33 mol % NaNO₃ and less than or equal to 55 mol% NaNO₃.